The present invention relates to an epoxy resin composition useful in the making of cast insulators employed in electric machines.
An expoxy resin combined with an acid anhydride cures to provide a product that has superior electrical, mechanical and chemical properties and which is extensively used as an epoxy resin cast insulator in electric machines including those employed in power transmission and distribution. If particularly good electrical and mechanical properties (e.g. high heat resistance) are required, cycloaliphatic epoxy resins having two or more epoxy groups in the molecule are used either alone or in combination with bisphenol A type epoxy resins. In order to improve the productivity of epoxy resin cast insulators using a smaller number of molds, a method commonly referred to as the superatmospheric gelling process which is capable of reducing the release time is currently employed. In this method, an epoxy resin blend of interest held within a cold pressurized tank is injected into a heated mold through a pipeline and a die head, while the mold is pressurized to compensate for any contraction of the resin being cured, thereby producing the desired casting within a shortened period of curing. The epoxy resin blend employed in this method must have a low viscosity and a long pot life within the cold pressurized tank, and must be capable of curing rapidly within the heated mold.
Those epoxy resins which exhibit low viscosities at low temperatures have low molecular weights and, hence, they exhibit on extremely high degree of shrinkage during curing and are highly likely to give cured products with sink marks and cracks. This problem is particularly serious with cycloaliphatic epoxy resins which are commonly employed for the purpose of providing improved heat resistance. In addition, epoxy resins that are highly reactive at elevated temperatures will also exhibit comparatively high reactivity at low temperatures and suffer from a shorter pot life. Common practice for dealing with these problems is to employ the superatmospheric gelling method with a view to preventing the occurrence of sink marks and cracks during the curing process and to use a latent accelerator for the purpose of extending the pot life of the resin blend. A problem arises, however, from the fact that epoxy resins of low molecular weights, such as cycloaliphatic epoxy resins, that will exhibit low viscosities at low temperatures are less resistant to thermal shock than the solid epoxy resins which are commonly employed in ordinary casting methods other than the superatmospheric gelling process.
One conventional method employed for improving the resistance of low-viscosity epoxy resins to thermal shock is to use them in combination with bisphenol A type epoxy resins, but the intended resistance to thermal shock cannot be attained without adding a large amount of bisphenol A type epoxy resin. Furthermore, the resulting epoxy resin composition has an excessively high viscosity at low temperatures and presents considerable difficulty in working operations at low temperatures. An alternative to the use of bisphenol A type epoxy resins is to add plasticity providing agents, such as high-molecular weight oligomers that has molecular weights within the range of from about 500 to 5,000 and which are comprised of polyester, polyether, polybutadiene or the like in the backbone chain. However, as the addition of these oligomers in increased, the viscosity of the epoxy resin is increased significantly while its heat resistance is considerably reduced. On the other hand, if the addition of such oligomers is insufficient, there is little possibility of improvement in the resistance of the produce against thermal shock. Plasticity providing agents such as those having polyamide in the backbone chain have the advantage that they will not substantially increase the viscosity of the resin blend but then, the resin blend incorporating such plasticity providing agent is highly reactive and has a shorter pot life.
In the superatmospheric gelling method, an epoxy resin blend having a low viscosity at low temperature is injected into a mold that is heated to a temperature higher than that of the resin blend. Within the mold, the viscosity of the resin blend is reduced temporarily to cause precipitation of the filler, giving rise to such problems as surface defects (e.g. flow marks) on the cured product and an appreciably high degree of unevenness in the distribution of filler's level in the cured product. This latter problem causes nonuniformity in the dielectric constant of the cured product and an insulator made of that product will have an unequal potential distribution in its cross section. An electric machine using this insulator will not only suffer from such electrical problems as reduced a.c. breakdown voltage but also has degraded mechanical properties such as low crack resistance.
In ordinary casting methods other than the super-atmospheric gelling process, the above-mentioned problems have hitherto been coped with by using an increased amount of filler or reducing its particle size so that the overall viscosity of the epoxy resin composition is sufficiently increased to prevent precipitation of the filler. However, this method is unable to produce a resin blend having a low viscosity at low temperature, and most of the problems associated with the drop in the viscosity of the resin blend that results from its injection into a mold having a higher temperature remain unsolved.
As explained above, the conventional epoxy resins are unable to have high resistance to both heat and thermal shock. In addition, when an epoxy resin blend having a low viscosity at low temperature is injected into a mold heated to a temperature higher than that of the blend, the filler will precipitate to cause not only surface flaws on the cured product but also nonuniformity or degradation in the properties of the final product.